1. Field of the Invention
This invention relates to probe fixtures used in connecting circuit boards or circuit panels to manual or automatic test equipment, and specifically to a single programmable probe which may be used to perform both functional and diagnostic testing, either static or dynamic. The invention further relates to a system of test equipment employing a plurality of probes in a fixture each probe being independently programmable, the method of interconnecting the unit under test to the stimulus and measurement equipment, and the program control therefor.
2. Prior Art
Test equipment, including probe fixtures, is available for performing functional and diagnostic testing of circuit boards and panels. Such equipment can perform both static and dynamic tests.
In order to perform such tests it is necessary to bring one or more probes into electrical contact with one or more terminals of the circuit board. Then, for example, by selectively applying electrical signals to one or more of the contacted terminals through the probes and by reading or analyzing the signal characteristics at other contacted terminals through the probes it is possible to functionally and diagnostically test the circuits on the board or panel.
A static test is one in which just DC signals are applied through the probes to the board and the DC performance characteristics of various circuit components are measured. In dynamic testing, AC signals are selectively applied as inputs and the AC characteristics of the various circuit components under test are measured. Functional testing is the testing of a circuit under its designed conditions. Diagnostic testing, as the name implies, is the process of fault isolation in a given circuit.
A dynamic functional test, then, is one in which AC inputs are applied through one or more probes to selected terminals and the AC outputs at other selected terminals are measured through the probes, without using any other extraneous probes, to determine if the outputs meet designed specifications. As known in the prior art, when high frequency signals were used in such tests, it was required to use a probe fixture having a minimum number of probes placed adjacent the test panel in order to avoid external loading and detuning of the circuit under test. This detuning problem was caused by the frequency characteristics of the probes themselves which resulted in parasitic inductances or capacitances in the tested circuit thereby altering its designed electrical characteristics. Faulty tests resulted.
Dynamic diagnostic testing, then, is the process of isolating a faulty circuit in an electronic module by tracing the AC signal sequentially from the input of one circuit to the output thereof and then to the input of the next circuit in line and then to its output, etc. until the faulty circuit is located. Such testing requires not only the probes contacting the initial input and final output terminals, as in functional testing, but also, the probes for contacting the various input and output terminals of the intermediate circuit components as well. Thus, a plurality of probes are required. In the prior art, since single independently programmable probes were not known until applicants' invention thereof, and due to the requirement to limit the number of probes which contacted the panels in order to avoid detuning of the circuit being dynamically tested, as aforesaid, the testing procedure required moving the test panel from the one probe fixture used for functional testing to another, or several other, probe fixtures for the diagnostic testing. Alternatively, it was known to mount the probes needed for each type of test to movable supports, vertically stack the supports one upon the other with the probes of the lower levels being received through holes in the upper level supports and then selectively reciprocate the levels of probes required by the particular test. Thus, the testing procedure was very laborious, time consuming and expensive in that numerous different or complex probe fixtures and test equipment were required.
The problem of detuning printed circuit boards is especially evident in the testing of analog and high speed logic modules which operate at high frequencies in the megahertz range. An example of such an analog module is the front end of an FM tuner. In such analog modules, operating voltages are maintained at a continuum of predetermined design levels. The effects of stray capacitances and inductances caused by external components are also sufficiently present in analog modules which use lower frequencies, as in the front end of an AM tuner which includes the RF and mixer stages, to cause faulty functional or diagnostic test results.
A typical prior art system in use at the time of this invention has a probe fixture commonly known as the "bed of nails" having a plurality of fixed probes to each of which is connected an electrical lead coupling to test equipment. A printed circuit board is fixed to a diaphram and placed over and aligned with the probe fixture, but spaced therefrom forming an air chamber between the fixture and the board. When it is desired to contact the probes against the board, the air chamber is evacuated, drawing the board into contact with the bed nails. As mentioned above, it is also known to have the panel fixedly mounted and to move the whole "bed of nails" or selected groups of "nails" into contact with the circuit board. Such systems are plagued with the shortcomings discussed above.